loop_schedule_deferred may leak its allocated Event if the Event does
not get processed (when Nvim exits).

It only happens on exit, so we tried to ignore it in .asan-blacklist,
but for that isn't working for unknown reasons.

    = ==22007==ERROR: LeakSanitizer: detected memory leaks
    = Direct leak of 88 byte(s) in 1 object(s) allocated from:
    =      0 0x4ef7a3 in malloc /local/mnt/workspace/tmp/ubuntu_rel/llvm/utils/release/final/llvm.src/projects/compiler-rt/lib/asan/asan_malloc_linux.cc:67:3
    =      1 0xfcafc1 in try_malloc /home/travis/build/neovim/neovim/build/../src/nvim/memory.c:88:15
    =      2 0xfcb1e9 in xmalloc /home/travis/build/neovim/neovim/build/../src/nvim/memory.c:122:15
    =      3 0xa6e75f in loop_schedule_deferred /home/travis/build/neovim/neovim/build/../src/nvim/event/loop.c:91:19
    =      4 0x18ab048 in tui_main /home/travis/build/neovim/neovim/build/../src/nvim/tui/tui.c:377:5
    =      5 0x18fe120 in ui_thread_run /home/travis/build/neovim/neovim/build/../src/nvim/ui_bridge.c:102:3
    =      6 0x7f09e5e58183 in start_thread /build/eglibc-ripdx6/eglibc-2.19/nptl/pthread_create.c:312
    = SUMMARY: AddressSanitizer: 88 byte(s) leaked in 1 allocation(s).

clang ASAN/LeakSanitizer error (observed in #7706):

    ==21832==ERROR: LeakSanitizer: detected memory leaks
    Direct leak of 56 byte(s) in 1 object(s) allocated from:
        0 0x511b26 in malloc (/home/travis/build/neovim/neovim/build/bin/nvim+0x511b26)
        1 0x1009a84 in try_malloc /home/travis/build/neovim/neovim/src/nvim/memory.c:87:15
        2 0x1009c44 in xmalloc /home/travis/build/neovim/neovim/src/nvim/memory.c:121:15
        3 0xaa8c36 in loop_schedule_deferred /home/travis/build/neovim/neovim/src/nvim/event/loop.c:89:19
        4 0x190856a in tui_main /home/travis/build/neovim/neovim/src/nvim/tui/tui.c:367:5
        5 0x1963d61 in ui_thread_run /home/travis/build/neovim/neovim/src/nvim/ui_bridge.c:106:3
        6 0x2b5d4190d183 in start_thread /build/eglibc-SvCtMH/eglibc-2.19/nptl/pthread_create.c:312

Possible explanation: During exit, `Loop.thread_events` may not get
flushed, so `loop_deferred_event()` is never called.

We could instead try to unwind `Loop.thread_events` during teardown, but
it seems lower-risk to just tell ASAN to ignore it.

Valgrind does not complain:
    $ while :; do { 2>valglog.txt valgrind ./build/bin/nvim -u NONE +q ; } ; if ! [ $? = 0 ] ; then break ; fi ; done

Also fixes buffer reusage in setmatches() and complete().

`lib/queue.h` implements a basic queue. `event/queue.c` implements
a specialized data structure on top of lib/queue.h; it is not a "normal"
queue.

Rename the specialized multi-level queue implemented in event/queue.c to
"multiqueue", to avoid confusion when reading the code.

Before this change one can eventually notice that "macros (uppercase
symbols) are for the normal queue, lowercase operations are for the
multi-level queue", but that is unnecessary friction for new developers
(or existing developers just visiting this part of the codebase).